This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In many fluid-handling systems, it is useful to adjust a fluid's flow rate. Often, fluids (e.g., a liquid, a gas, or combination thereof) enter the system at a relatively high pressure before flowing to lower pressure regions of the system. The flow rate driven by the resulting pressure drop may be greater than desired. High flow rates may erode components, generate unpleasant noise, and deliver greater volumes of fluid than downstream components are equipped to optimally process.
To adjust flow rates, many fluid-handling systems include choke valves. These valves typically include a movable valve member that translates over an opening through which the fluid flows. By shifting the position of the valve member relative to the opening, the size of the opening may be increased or decreased, and the flow rate of the fluid may be adjusted. In some types of choke valves, the valve member can close the opening and seal against a seat, thereby dropping the flow rate to zero or near zero and closing the choke valve.
Many conventional choke valves are difficult to control when they are near the closed position, e.g., within the last 5 to 10 percent of the valve member's travel. As the valve member opens from the closed position, fluid flow often increases relatively rapidly as an initial gap is formed. As the opening grows, the rate of change in the flow rate stabilizes, and flow is more easily controlled by adjusting the position of the valve member. The initial jump in the flow rate, however, makes controlling low flow rates difficult, as relatively small changes in the position of the valve member may have a relatively large impact on the flow rate. Choke valves are characterized by the range of flow rates over which they are controllable. This property is referred to as “rangeability.” The rapid increase in flow rate as the valve member is initially opened decreases rangeability, as deviations in the position of the valve member may produce relatively large shifts in the flow rate.